A voltage regulator is operative to maintain a level output voltage despite variations in power supply voltage or current drawn by a load. These regulators typically output a relatively high voltage, for example in the hundreds of volts, which is then used to power or charge a load. High voltage regulators can be implemented in various forms. These circuits receive as an input a relatively low voltage, typically five volts or less, and output a high voltage. Applications of these high voltage regulators include charging a photoflash capacitor, such as those commonly used in cameras.
In general, the high voltage regulators use a transformer or possibly another similar device to increase the supply voltage to the desired high voltage for driving the load. In general, the transformer has a relatively high turns ratio of ten or greater for typical photoflash applications.
FIG. 1 shows a prior art high voltage regulator used for charging a photoflash capacitor C1. A transformer T1 is used to transform an input power supply voltage VCC into a high voltage output. The current on the secondary side of the transformer T1 flows through a diode D1 to charge the capacitor C1. Various integrated circuitry is used to control the operation of the high voltage regulator. For example, a control section responsive to an enable pin and a charge request pin controls a power switch for delivering power to the transformer T1. In order to determine whether or not the output voltage of the secondary winding of the transformer T1 is at the proper output voltage, the prior art of FIG. 1 measures the back electromagnetic field (EMF) of the primary winding of the transformer T1. This can be used to deduce the output voltage on the secondary winding.
As seen in FIG. 1, after power is applied and a charge request is set, the voltage regulator periodically turns on the power switch until an internal set current is reached in the switch. The primary winding of the transformer T1 inductively kicks up past the positive supply VCC creating a proportionate change in voltage on the secondary. As the voltage on the capacitor C1 increases, the “kicked voltage” (also referred to as “back EMF”) on the primary winding of the transformer T1 also increases. The voltage regulator stops charging when the back EMF detected on the primary winding of the transformer T1 reaches a preset level which corresponds to a desired output voltage level. The detection is performed by the voltage comparator in conjunction with the voltage reference. The prior art method of FIG. 1 has an inherent inaccuracy due to the transformer's effective turns ratio.
A second prior art approach is shown in FIG. 2 where a resistor divider formed by resistors R1 and R2 is used. However, this configuration has a disadvantage of leaking charge off of the capacitor C1. This is a disadvantage if the output is meant to hold its voltage between charging cycles. Still another prior art method shown in FIG. 3 uses the resistor divider but with an extra diode from the transformer output. Both methods have either the problem of very slow response for high impedance dividers or the problem of too much loading on the output for low impedance dividers. An additional capacitor will hold the peak long enough for a high impedance divider to measure the output voltage, but high voltage capacitors are costly.
The present invention provides an improved high voltage regulator.